In recent years, various methods for utilizing exhaust heat for environmental protection have been proposed. For example, there is proposed a method of driving a generator using a gas engine as a power source, recovering exhaust heat from the engine with an exhaust heat recovery heat exchanger, storing the recovered exhaust heat in a hot water tank as hot water, and supplying the hot water from the hot water tank to an external facility (see, for example, PTL 1). There is also proposed a tri-generation system that receives the supply of fuel containing carbon, recovers, with an exhaust heat recovery unit, exhaust heat from a motor for driving a generator, supplies the heat to a greenhouse for protected horticulture, and supplies carbon oxide generated by combustion of the fuel to the greenhouse for protected horticulture as well (see, for example, PTL 2). PTL 2 also proposes to provide a hot water tank to temporarily store heat and enable heating of the greenhouse even when the motor is stopped.